Tells if an error occurred during an operation on the stream.
Since EOF is used to report errors by some functions, it's often better to
use the pack_feof() function to check explicitly for end of file and
pack_ferror() to check for errors. Both functions check indicators that
are part of the internal state of the stream to detect correctly the
Returns nonzero if the error indicator for the stream is set, meaning
that an error has occurred during a previous operation on the stream.
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